No one wants to lose power on a manufacturing line, as the results can be anywhere from a minor annoyance to a significant loss of ware and possible equipment damage. Unfortunately, uninterruptible power supply (UPS) sizing and specification for factory automation often gets overlooked until there is a painful power loss incident.
The task of planning how and what to protect with factory UPS systems requires a thorough understanding and prioritization of the assets in the line, their power demands, the costs involved, the sensitivity of equipment to voltage fluctuations and whether there is a backup power source that can step in to keep things running.
We have covered the types and sizes of UPS systems in the past, which range from the smallest of systems that run for a few seconds allowing capturing and storage of the last running data, to full-blown systems that are capable of supporting entire production lines and all the necessary peripheral equipment.
The latter case, because of the huge cost and infrastructure requirements, is typically used only for the highest value, most critical lines, and not the norm for most UPS system design that we encounter routinely.
The vast majority of installed UPS systems will protect only selected control equipment rather than full production loads, and the metrics used for helping us design a suitable UPS system are a little different than in information technology (IT) or facility-scale UPS sizing. Let’s look at some key concepts and rules of thumb for specifying smaller UPS systems in a typical production line.
1. Define the protected load clearly
Core candidates include programmable logic controllers (PLCs), motion controllers, industrial PCs, human-machine interfaces (HMIs), Internet switches and routers, safety relays and data loggers. Avoid powering large drives, motors, heaters or anything that spikes the power load; they can overwhelm small UPS systems. Additionally, avoid putting “nice to have” devices, such as large monitors or printers, on the same UPS as critical controllers.
Rule of thumb: Protect only what you need to shut down gracefully or maintain critical data and logs for later recovery. Each UPS should support a clearly defined critical load segment with no hidden surprises.
2. Size the system for the appropriate runtime, not just the load
Unlike IT systems, you usually don’t need hours of runtime, just enough for an orderly shutdown, a ride-through of short brownouts or the transfer to a backup generator.
Rule of thumb: Five to 15 minutes of runtime is sufficient for most automation controllers. Don’t oversize runtime, as you will add maintenance and cost for batteries that never get used.
3. Voltage strategy
Decide whether to back up the ac mains to the control panel, typically 120/230 Vac, or whether to provide direct 24 Vdc UPS to controllers.
Alternating-current UPS, such as IT-style rack systems, are easy to source and flexible, and can power any 24 Vdc supply in the panel. But DIN-rail-type 24 Vdc industrial UPS components are specifically designed for automation and have smaller footprints.
Rule of thumb: For powering critical controllers, a DIN-rail dc UPS is often better.
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4. Communications and alarming
It is important to not just provide backup power, but make it visible. Many industrial UPS units offer relay contacts or fieldbus outputs. This ties the UPS status into your PLC/HMI so operators know when the system is running on battery, when runtime is low or when battery health is poor.
Rule of thumb: If the system is critical, make UPS events part of your supervisory control and data acquisition (SCADA)/alarm system, not just hidden in a box.
5. Battery chemistry and environment
Standard lead-acid batteries degrade faster in hot control cabinets, whereas Lithium-ion UPS options are increasingly available for automation, providing higher cycle life, better temperature tolerance and smaller footprint. Although IT UPS units can work fine in a panel, industrial UPS systems have wider temperature ratings, DIN-rail mounting and conformal coating if in humid or dusty environments.
Rule of thumb: In a hot factory environment, where the temperature can be greater than 30 °C, lithium-ion will pay off in reliability. For plant-floor deployment, go industrial UPS first and IT UPS second.
6. Maintenance & testability
UPS systems are notorious for failing silently due to aging batteries.
Rule of thumb: Replace batteries every three to five years for lead-acid, eight to 10 years for lithium-ion systems. Additionally, schedule annual discharge tests to validate runtime.
7. Generator coordination, if applicable
If your facility has backup generator sets, your UPS runtime only needs to cover the genset start and transfer time, typically a minute or two plus a small buffer, or around five minutes max.
Rule of thumb: Five minutes of runtime is plenty if a generator exists. Without a generator, aim for 15 minutes for orderly shutdown.
By focusing on powering only the brains of the system, not the brawn, the UPS power demand scales down to become much more affordable and realistic. But keep in mind that industrial UPS devices offer greater environmental specs and visibility to the control system, adding to the reliability of a, hopefully, seldom-used component of the factory control system.
